1. Field of the Invention
The present invention relates broadly to the art of rendering and maintaining a surface of a substrate highly hydrophilic. More particularly, the present invention relates to the antifogging art wherein the surface of a transparent substrate such as a mirror, lens and sheet glass is made highly hydrophilic to thereby prevent fogging of the substrate or formation of water droplets. This invention is also concerned with the art wherein the surface of a building, windowpane, machinery or article is rendered highly hydrophilic in order to prevent fouling of, to permit self-cleaning of or to facilitate cleaning of the surface. This invention also relates to a hydrophilifiable member having a surface layer which is capable of having an extremely small contact angle with water, a method for rendering the member hydrophilic, a method for forming a hydrophilifiable surface layer, and a coating composition for forming a hydrophilifiable surface layer.
2. Description of the Prior Art
It is often experienced that, in the cold seasons, windshields and window-glasses of automobiles and other vehicles, windowpanes of buildings, lenses of eyeglasses, and cover glasses of various instruments are fogged by moisture condensate. Similarly, in a bathroom or lavatory, it is often encountered that mirrors and eyeglass lenses are fogged by steam.
Fogging of the surface of an article results from the fact that, when the surface is held at a temperature lower than the dew point of the ambient atmosphere, condensation of moisture in the ambient air takes place to form moisture condensate at the surface.
If the condensate particles are sufficiently fine and small so that the diameter thereof is on the order of one half of the wavelength of the visible light, the particles cause scattering of light whereby window-glasses and mirrors become apparently opaque thereby giving rise to a loss of visibility.
When condensation of moisture further proceeds so that fine condensate particles are merged together to grow into discrete larger droplets, the refraction of light taking place at the interface between the droplets and the surface and between the droplets and the ambient air causes the surface to be blurred, dimmed, mottled, or clouded. As a result, an image viewed through a transparent article such as sheet glass may become distorted, or the reflected image in a mirror may be disturbed.
Similarly, when windshields and window-glasses of vehicles, windowpanes of buildings, rearview mirrors of vehicles, lenses of eyeglasses, or shields of masks or helmets are subjected to rain or water splash so that discrete water droplets are adhered to the surface, their surface is blurred, dimmed, mottled, or clouded, resulting in the loss of visibility.
The term xe2x80x9cantifoggingxe2x80x9d as used herein and in the appended claims is intended to mean broadly the art of preventing or minimizing occurrence of optical trouble resulting from fogging, growth of condensate droplets or adherent water droplets mentioned above.
The antifogging art can significantly affect safety and efficiency in a variety of setting. For example, the safety of vehicles and traffic can be undermined if the windshields, window-glasses or rearview mirrors of vehicles are fogged or blurred. Fogging of endoscopic lenses and dental mouth mirrors may hinder proper and accurate diagnosis, operation and treatment. If cover glasses of measuring instruments are fogged, a reading of data will become difficult.
The windshields of automobiles and other vehicles are normally provided with windshield wipers, defrosting devices and heaters so as to avoid visibility problems, which arise particularly in the cold seasons and under rainy conditions. However, it is not commercially feasible to install this equipment on the side windows of a vehicle, or on the rearview mirrors arranged outside of the vehicle. similarly, it is difficult, if possible at all, to mount such antifogging equipment on windowpanes of buildings, lenses of eyeglasses and endoscopes, dental mouth mirrors, shields of masks and helmets, or cover glasses of measuring instruments.
As is well-known, a simple and convenient antifogging method conventionally used in the art is to apply onto a surface an antifogging composition containing either a hydrophilic compound such as polyethylene glycol or a hydrophobic or water-repellent compound such as silicone. However, the disadvantage of this method is that the antifogging coating thus formed is only temporary in nature and is readily removed when rubbed or washed with water so that its effectiveness is prematurely lost.
Japanese Utility Model Kokai Publication No. 3-129357 (Mitsubishi Rayon) discloses an antifogging method for a mirror wherein the surface of a substrate is provided with a polymer layer and the layer is subjected to irradiation by ultraviolet light, followed by treatment with an aqueous alkaline solution to thereby form acid radicals at a high density whereby the surface of the polymer layer is rendered hydrophilic. Again, however, it is believed that, according to this method, the hydrophilic property of the surface is degraded as time elapses because of adherent contaminants so that the antifogging function is lost over time.
Japanese Utility Model Kokai Publication No. 5-68006(Stanley Electric) discloses an antifogging film made of a graftcopolymer of an acrylic monomer having hydrophilic groups and a monomer having hydrophobic groups. The graftcopolymer is described as having a contact angle with water of about 50xc2x0. It is therefore believed that this antifogging film does not exhibit a sufficient antifogging capability.
Isao Kaetsu xe2x80x9cAntifogging Coating Techniques for Glassxe2x80x9d, Modern Coating Techniques, pages 237-249, published by Sogo Gijutsu Center (1986), describes various antifogging techniques used in the prior art. The author Mr. Kaetsu nevertheless reports that the prior art antifogging techniques, which consist of rendering a surface hydrophilic, suffer from significant problems which must be overcome in reducing them to practice, and, further reports that the conventional antifogging coating techniques seemingly come up against a barrier.
Accordingly, an object of the invention is to provide an antifogging method which is capable of realizing a high degree of visibility in a transparent substrate such as a mirror, lens or glass.
Another object of the invention is to provide an antifogging method wherein the surface of a transparent substrate such as a mirror, lens or glass is maintained highly hydrophilic for an extended period of time.
Still another object of the invention is to provide an antifogging method wherein the surface of a transparent substrate such as a mirror, lens and glass is almost permanently maintained highly hydrophilic.
A further object of the invention is to provide an antifogging coating which has an improved durability and abrasion resistance.
Another object of the invention is to provide an antifogging coating which can be readily applied onto a surface requiring antifogging treatment.
Yet another object of the invention is to provide an antifogging transparent substrate such as a mirror, lens or glass, as well as a method of making such an antifogging transparent substrate, wherein the substrate surface is maintained highly hydrophilic for an extended period of time to thereby provide a high degree of antifogging property for an extended period.
In the fields of architecture and painting, it has been pointed out that growing environmental pollution tends to inadvertently accelerate fouling, contamination or soiling of exterior building materials, including outdoor buildings themselves and the coatings thereon.
In this regard, air-borne grime and dust particles are allowed under fair weather conditions to fall and deposit on roofs and outer walls of buildings. When it rains, the deposits are washed away by rainwater and are caused to flow along the outer walls of the buildings. Furthermore, air-borne grime is captured by rain and is carried onto surfaces (such as outer walls) of outdoor structures and buildings, where the grime may flow along or down the surface. For these reasons, contaminant substances are caused to adhere onto the surface along the paths of rainwater. As the surface is dried, a striped pattern of dirt, stain or smudge will appear on the surface.
The dirt or stain thus formed on the exterior building materials and exterior coatings consists of contaminant substances which include combustion products such as carbon black, city grime, and inorganic substances such as clay particles. The diversity of the fouling substances is considered to make the antifouling countermeasures complicated (Yoshinori KITSUTAKA, xe2x80x9cAccelerated Test Method For soiling on Finishing Materials of External Wallsxe2x80x9d, Bulletin of Japan Architecture Society, vol. 404 (October 1989), pages 15-24). Hitherto, it has been commonly considered in the art that water-repellent paints such as those containing polytetra-fluoroethylene (PTFE) are desirable to prevent fouling or soiling of exterior building materials and the like. Recently, however, it is pointed out that, in order to cope with city grime containing a large amount of oleophilic components, it is rather desirable to render the surface of coatings as hydrophilic as possible (xe2x80x9cHighpolymerxe2x80x9d, vol. 44, May 1995, page 307).
Accordingly, it has been proposed in the art to coat a building with a hydrophilic graftcopolymer (Newspaper xe2x80x9cDaily Chemical Industryxe2x80x9d, Jan. 30, 1995). Reportedly, the coating film presents a hydrophilicity of 30-40xc2x0 in terms of the contact angle with water.
However, in view of the fact that inorganic dusts, which may typically be represented by clay minerals, have a contact angle with water ranging from 20 to 50xc2x0 (so that they have affinity for graftcopolymer having a contact angle with water of 30-40xc2x0), it is considered that such inorganic dusts are apt to adhere to the surface of the graftcopolymer coating and, hence, the coating is not able to prevent fouling or contamination by inorganic dusts.
Also available in the market are various hydrophilic paints which comprise acrylic resin, acryl-silicone resin, aqueous silicone, block copolymers of silicone resin and acrylic resin, acryl-styrene resin, ethylene oxides of sorbitan fatty acid, esters of sorbitan fatty acid, acetates of urethane, cross-linked urethane of polycarbonatediol and/or polyisocyanate, or cross-linked polymers of alkylester polyacrylate. However, since the contact angle with water of these hydrophilic paints is as large as 50-70xc2x0, they are not suitable to effectively prevent fouling by city grimes which contain large amount of oleophilic components.
Accordingly, a further object of the invention is to provide a method for rendering a surface of a substrate highly hydrophilic and antifouling.
Another object of the invention is to provide a method wherein the surface of buildings, window glasses, machinery or articles is rendered highly hydrophilic to thereby prevent fouling of or to permit self-cleaning of or to facilitate cleaning of the surface.
Yet another object of the invention is to provide a highly hydrophilic antifouling substrate, as well as a method of making thereof, which is adapted to prevent fouling of or to permit self-cleaning of or to facilitate cleaning of the surface.
In certain apparatus, formation of moisture condensate on a surface thereof often hampers operation of the apparatus when condensate has grown into droplets. In heat exchangers, for example, the heat exchanging efficiency would be lowered if condensate particles adhering to radiator fins have grown into large droplets.
Accordingly, another object of the invention is to provide a method for preventing adherent moisture condensate from growing into larger water droplets wherein a surface is made highly hydrophilic to thereby permit adherent moisture condensate to spread into a water film.
The present inventors have found that, upon photoexcitation, a surface of a photocatalyst is rendered highly hydrophilic. Surprisingly, it has been discovered that, upon photoexcitation of photocatalytic titania with ultraviolet light, the surface thereof is rendered highly hydrophilic to the degree that the contact angle with water becomes less than 10xc2x0, more particularly less than 5xc2x0, and even reached about 0xc2x0.
Based on the foregoing findings, the present invention provides, broadly, a method for rendering a surface of a substrate highly hydrophilic, a substrate having a highly hydrophilic surface and a method of making such a substrate. According to the invention, the surface of the substrate is coated with an abrasion-resistant photocatalytic coating comprised of a photocatalytic semiconductor material.
Upon irradiation for a sufficient time with a sufficient intensity of a light having a wavelength which has an energy higher than the bandgap energy of the photocatalytic semiconductor, the surface of the photocatalytic coating is rendered highly hydrophilic to exhibit a super-hydrophilicity. The term xe2x80x9csuper-hydrophilicityxe2x80x9d or xe2x80x9csuper-hydrophilicxe2x80x9d as used herein refers to a highly hydrophilic property (i.e., water wettability) of less than about 10xc2x0 in terms of the contact angle with water. Similarly, the term xe2x80x9csuperhydrophilificationxe2x80x9d or xe2x80x9csuperhydrophilifyxe2x80x9d refers to rendering a surface highly hydrophilic to the degree that the contact angle with water becomes less than about 10xc2x0. It is preferred that a super-hydrophilic drophilic surface have a water wettability of less than about 5xc2x0.
The process of superhydrophilification of a surface resulting from photoexcitation of a photocatalyst cannot be explained presently with any certainty. seemingly, photocatalytic superhydrophilification is not necessarily identical with photodecomposition of a substance arising from photocatalytic redox process known hitherto in the field of photocatalyst. In this regard, the conventional theory admitted in the art regarding the photocatalytic redox process was that electron-hole pairs are generated upon photoexcitation of the photocatalyst, the electrons thus generated acting to reduce the surface oxygen to produce superoxide ions (O2xe2x88x92), the holes acting to oxidize the surface hydroxyl groups to produce hydroxyl radicals (xc2x7OH), these highly active oxygen species (O2xe2x88x92and xc2x7OH) then acting to decompose a substance through redox process.
However, it seems that the superhydrophilification phenomenon provoked by a photocatalyst is not consistent, in at least two aspects, with the conventional understanding and observation regarding the photocatalytic decomposition process of substances. First, according to a theory widely accepted hitherto, it has been believed that, in a certain photocatalyst such as rutile and tin oxide, the energy level of the conduction band is not high enough to promote the reduction process so that the electrons photoexcited up to the conduction band remain unused and become excessive whereby the electron-hole pairs once generated by photoexcitation undergo recombination without contributing in the redox process. In contrast, the present inventors have observed that the super-hydrophilification process by a photocatalyst takes place even with rutile and tin oxide, as described later.
Secondly, the conventional wisdom was that the decomposition of substances due to photocatalytic redox process is not developed unless the thickness of a photocatalytic layer is greater than at least 100 nm. Conversely, the present inventors have found that photocatalytic superhydrophilification occurs even with a photocatalytic coating having a thickness on the order of several nanometers.
Accordingly, it is believed (though it cannot be confirmed with certainty) that the superhydrophilification process caused by a photocatalyst is a phenomenon somewhat different from photodecomposition of substances resulting from the photocatalytic redox process. As described later, it has been observed that superhydrophilification of a surface does not occur unless a light having an energy higher than the band gap energy of the photocatalyst is irradiated. It is considered that, presumably, the surface of a photocatalytic coating is rendered superhydrophilic as a result of water being chemisorbed thereon in the form of hydroxyl groups (OHxe2x88x92) under the photocatalytic action of the photocatalyst.
Once the surface of the photocatalytic coating has been made highly hydrophilic upon photoexcitation of the photocatalyst, the hydrophilicity of the surface will be sustained for a certain period of time even if the substrate is placed in the dark. As time elapses, the superhydrophilicity of the surface will be gradually lost because of contaminants adsorbed on the surface hydroxyl groups. However, the superhydrophilicity will be restored when the surface is again subjected to photoexcitation.
To initially superhydrophilify the photocatalytic coating, any suitable source of light may be used which has a wavelength of an energy higher than the band gap energy of the photocatalyst. In the case of those photocatalysts such as titania for which the photoexciting wavelength is in the ultraviolet range of the spectrum, the ultraviolet light contained in the sunlight may advantageously be used as the photoexciting light source if the sunlight impinges upon the substrate coated by the photocatalytic coating. When the photocatalyst is to be photoexcited indoors or at night, an artificial light source may be used. In the case where the photocatalytic coating is made of silica blended titania as described later, the surface thereof can readily be rendered hydrophilic even by a weak ultraviolet radiation contained in the light emitted from a fluorescent lamp.
After the surface of the photocatalytic coating has once been superhydrophilified, the superhydrophilicity may be maintained or renewed by a relatively weak light. In the case of titania, for example, maintenance and restoration of the superhydrophilicity may be accomplished to a satisfactory degree even by a weak ultraviolet light contained in the light of indoor illumination lamps such as fluorescent lamps.
The photocatalytic coating of the present invention exhibits super-hydrophilicity even if the thickness thereof is made extremely small. It presents a sufficient hardness when made, in particular, from a photocatalytic semiconductor material comprising a metal oxide. Therefore, the present photocatalytic coating may have an adequate durability and abrasion resistivity.
Superhydrophilification of a surface may be utilized for various applications. In one aspect of the invention, this invention provides an antifogging method for a transparent member, or provides an antifogging transparent member, or provides a method of making an antifogging member. According to the invention, a transparent member is prepared, and the surface of the transparent member is coated with a photocatalytic coating.
The transparent member may include a mirror such as a rearview mirror for a vehicle, a bathroom or lavatory mirror, a dental mouth mirror, or a road mirror; a lens such as an eyeglass lens, optical lens, photographic lens, endoscopic lens, or light projecting lens; a prism; a windowpane for a building or control tower; a windowpane for a vehicle such as an automobile, railway vehicle, aircraft, watercraft, submarine, snowmobile, ropeway gondola, pleasure garden gondola and spacecraft; a windshield for a vehicle such as an automobile, railway vehicle, aircraft, watercraft, submarine, snowmobile, motorcycle, ropeway gondola, pleasure garden gondola and spacecraft; a shield for protective or sporting goggles or mask including diving mask; a shield for a helmet; a show window glass for chilled foods; or a cover glass for a measuring instrument.
Upon subjecting the transparent member provided with the photocatalytic coating to irradiation by a light to thereby photoexcite the photocatalyst, the surface of the photocatalytic coating will be superhydrophilified. Thereafter, in the event that moisture in the air or steam undergoes condensation, the condensate will be transformed into a relatively uniform film of water without forming discrete water droplets. As a result, the surface will be free from the formation of a light diffusing fog.
Similarly, in the event that a windowpane, a rearview mirror of a vehicle, a windshield of a vehicle, eyeglass lenses, a helmet shield, or other substrate is subjected to a rainfall or a splash of water, the water droplets adhering onto the surface will be quickly spread over into a uniform water film thereby preventing formation of discrete water droplets which would otherwise hinder visibility through, or reflection from, the substrate.
Accordingly, a high degree of view and visibility is secured so that the safety of vehicle and traffic is secured and the efficiency of various activities improved.
In another aspect, this invention provides a method for self-cleaning a surface of a substrate wherein the surface is superhydrophilified and is self-cleaned by rainfall. This invention also provides a self-cleaning substrate and a method of making thereof.
The substrate may be any of a variety of articles, including an exterior member, window sash, structural member, or windowpane of a building; an exterior member or coating of a vehicle such as automobile, railway vehicle, aircraft, and watercraft; an exterior member, dust cover or coating of a machine, apparatus or article; and an exterior member or coating of a traffic sign, various display devices, and advertisement towers, that are made, for example, of metal, ceramics, glass, plastics, wood, stone, cement, concrete, a combination thereof, a laminate thereof, or other materials. The surface of the substrate is coated with the photocatalytic coating.
Since the building, or machine or article disposed outdoors, is exposed to the sunlight during the daytime, the surface of the photocatalytic coating will be rendered highly hydrophilic. Furthermore, the surface will occasionally be subjected to rainfall. Each time the superhydrophilified surface receives a rainfall, dust and grime and contaminants deposited on the surface of the substrate will be washed away by rain whereby the surface is self-cleaned.
As the surface of the photocatalytic coating is rendered highly hydrophilic to the degree that the contact angle with water becomes less than about 10xc2x0, preferably less than about 5xc2x0, particularly equal to about 0xc2x0, not only city grime containing large amounts of oleophilic constituents but also inorganic dusts such as clay minerals will be readily washed away from the surface. In this manner, the surface of the substrate will be self-cleaned and kept clean to a high degree under the action of nature. This will permit, for instance, to eliminate or largely reduce cleaning of windowpanes of towering buildings.
In still another aspect, this invention provides an antifouling method for a building, window glass, machine, apparatus, or article wherein the surface thereof is provided with a photocatalytic coating and is rendered highly hydrophilic to prevent fouling.
The surface thus superhydrophilified will prevent contaminants from adhering to the surface when rainwater which is laden with contaminants originating from air-borne dust and grime flows down along the surface. Therefore, in combination with the above-mentioned self-cleaning function performed by rainfall, the surface of the building and the like may be maintained in a high degree of cleanliness for an extremely long period of time.
In a further aspect of the invention, a photocatalytic coating is provided on a surface of an apparatus or article, such as an exterior or interior member of a building, or a windowpane, household, toilet bowl, bath tub, wash basin, lighting fixture, kitchenware, tableware, sink, cooking range, kitchen hood, or ventilation fan, said apparatus or article being made from metal, ceramics, glass, plastics, wood, stone, cement, concrete, a combination thereof, a laminate thereof, or other materials, and the surface is photoexcited as required.
When these articles which are fouled by oil or fat are soaked in, wetted with or rinsed by water, fatty dirt and contaminants will be released from the superhydrophilified surface of the photocatalytic coating and will be readily removed therefrom. Accordingly, for example, a tableware fouled by oil or fat may be cleansed without resort to a detergent.
In another aspect, this invention provides a method for preventing growth of condensate droplets adhering to a substrate or for causing adherent water droplets to spread into a uniform water film. To this end, the surface of the substrate is coated with a photocatalytic coating.
Once the surface of the substrate has been super-hydrophilified upon photoexcitation of the photocatalytic coating, moisture condensate or water droplets that have come to adhere to the surface will be spread over the surface to form a uniform aqueous film. By applying this method, for example, to radiator fins of a heat exchanger, it is possible to prevent fluid passages for a heat exchange medium from being clogged by condensate; thus the present invention may be used to enhance the heat exchange efficiency. Also, when this method is applied to a mirror, lens, windowpane, windshield, pavement, or other such surface, it is possible to promote drying of the surface after wetting with water.
The present inventors have further discovered that hydrophilification of a surface layer made of a photocatalyst results from water molecules being physisorbed onto the surface under the photocatalytic action of the photocatalyst.
Based on this discovery, the present invention further provides a method and a composite wherein a substrate is coated with a surface layer comprised of a photocatalyst and wherein upon photoexcitation of the photocatalyst the molecules of water are physisorbed by hydrogen bonding onto the surface layer to thereby form a layer of physisorbed water of a high density.
As a layer of physisorbed water is formed on the surface of the photocatalytic layer, the surface is readily hydrophilified to a high degree. Due to the presence of the layer of physisorbed water, the hydrophilicity of the surface is maintained for a long period of time even after photoexcitation is discontinued, thereby minimizing the loss of hydrophilicity over time. Moreover, when the photocatalyst is photoexcited again, the hydrophilicity of the surface is readily recovered within a short period of time of irradiation or with a weak irradiation intensity.
These features and advantages of the invention as well as other features and advantages thereof will become apparent from the following description.